This is good advice. You will connect through whatever carrier is available, even if your own is not. You will also bump others users off the cell as emergency numbers have priority. So you will be much more likely to get through. You will NOT get through if there is no coverage though. For this you need a satellite phone, a UHF radio or an epirb. http://www.theultralighthiker.com/emergency-cb-radios/ See Snopes: ‘Calling 112 on your cell phone will (in some parts of the world, primarily Europe) connect you to local emergency services, even if you are outside your provider’s service area (i.e., even if you are not authorized to relay signals through the cell tower that handles your call), and many cell phones allow the user to place 112 calls even if the phone lacks a SIM card or its keypad is locked. However, the 112 number does not have (as is sometimes claimed) special properties that enable callers to use it in areas where all cellular signals are blocked (or otherwise unavailable).’ http://www.snopes.com/inboxer/household/cellphones.asp
Note: You need less signal to send an SMS than a voice call: This may get a message out when other means will not.
You may be able to extend battery life as an an option available on some brands of cell phone (such as Nokia) for Half Rate Codec, which provides about 30% more talk time on a battery charge at the expense of lower sound quality. This option is enabled by pressing the sequence *#4720#. A far better plan to conserve battery life is to put your phone on Flight Mode, or switch it off! It will last for MANY days.